1. Field of the Invention
The present invention relates to a wide-band horizontal linearity correction circuit and, more particularly, to a correction circuit which corrects wide-band horizontal linearity in a display using hysteresis of its line transformer.
2. Description of the Related Art
A display is a computer peripheral device which exhibits a signal received from a computer as a visual image. The display includes a microprocessor for receiving a video signal, horizontal and vertical sync signals for forming a picture from a video card in a host computer to generate a control signal for controlling the display picture. The display also includes vertical and horizontal deflection circuits for receiving the horizontal and vertical sync signals and for effecting horizontal and vertical deflections so that an electron beam, generated by an electron gun of a CRT, is sequentially deflected from the upper left corner of the CRT to its lower right hand corner by a deflection yoke to thereby reproduce an image. The display also includes a high voltage circuit for supplying a high voltage to an anode of the CRT, a video pre-amplifier, a low-voltage amplifier for amplifying the video signal from the video card to maintain a predetermined voltage level and a main video amplifier for amplifying the signal from the video pre-amplifier.
With this display, images are created as the electron beam is projected onto the display fluorescent screen. The deflection circuit deflects the electron beam.
The horizontal circuit of the display includes a horizontal oscillator circuit for generating a saw-tooth wave using the charging/discharging characteristics of a capacitor, a horizontal drive circuit for amplifying the output pulse of the horizontal oscillator circuit to supply sufficient base current to turn a horizontal output transistor on and off and for performing waveform correction, a horizontal output circuit for generating a saw-tooth current signal using a deflection coil and the switching operation of the horizontal output transistor, a horizontal regulator circuit for transmitting a signal for controlling the horizontal size to the horizontal output circuit, a high voltage drive circuit for generating a high voltage which is supplied to the anode of the CRT, a high voltage output circuit for amplifying the output voltage of the high voltage drive circuit and for converting it into the driving voltage of a flyback transformer, a high voltage regulator circuit for controlling the output cycle of the high voltage output circuit, and an X-ray protection circuit for stopping the horizontal oscillator circuit when the output voltage of the flyback transformer exceeds a predetermined level.
The horizontal output circuit includes a horizontal output transistor whose base receives a horizontal drive pulse, a damper diode, a resonance capacitor, a horizontal deflection yoke, and a line coil and S-correction capacitor which are connected to the collector of the horizontal output transistor. The horizontal output transistor is switched by the horizontal drive pulse and a large amount of current is supplied to the horizontal deflection yoke through the S-correction capacitor to generate a horizontal saw-tooth current signal. In order to maintain linearity of the horizontal saw-tooth current signal according to the drive frequencies, the line coil circuit and S-correction capacitor are connected to the horizontal deflection yoke.
When the frequency of a horizontal sync signal changes, the linearity changes according to the drive frequencies so that a relay is used to connect a separate linearity correction coil in parallel with the line coil as required at different frequencies. This relay is switched on and off by a microprocessor. However, because of the high voltage and large amount of current involved, the relay must be substantial so as to be able to handle the high current and voltage. Furthermore, noise is created when the relay is switched. In addition, the linearity properties depend on the deviation of the linearity correction coil and the horizontal deflection yoke.
The patents to Suzuki, Sahara et al., Truskalo, Gibbs et al., and Yang et al., U.S. Pat. Nos. 3,944,879, 3,968,402, 5,138,238, 5,179,322 and 5,434,483, respectively entitled Pin Cushion Distortion Correction Circuit, Raster Distortion Correcting Circuit, Arrangement For Correcting Beam Landing Location Error In A Video Display, Linearity Correction Apparatus, and Automatic Compensation Circuit For The Horizontal Deflection Of A Multifrequency Computer Monitor, each disclose correction circuits utilizing transformers connected in series with the horizontal deflection yoke and S-correction capacitor. However, none of these references teaches or suggests the use of a microprocessor for supplying a linearity correction control signal to a coil driver driving such a transformer as in the present invention.
The Jackson et al. patent, U.S. Pat. No. 5,475,286, entitled Deflection Waveform Correction Circuit, discloses an S-correction circuit rather than the horizontal linearity circuit of the present invention.
The Ando et al. and Bando patents, U.S. Pat. Nos. 4,754,204 and 5,517,090, respectively entitled Digital Apparatus For Convergence Correction, and S-Correction Capacitor Switching For A Display Monitor, disclose correction circuits utilizing digital processors for generating correction signals. However, Ando et al. utilizes a correction coil which is separate from the deflection coil while Bando varies the value of the S-correction capacitor rather than the inductance of the series circuit consisting of the horizontal deflection coil and S-correction capacitor.
The following additional patents each disclose features in common with the present invention but are not as pertinent as the patents discussed in detail above: U.S. Pat. No. 5,434,484 to Murakami, entitled Raster Left-And Right Distortions Correcting Apparatus, U.S. Pat. No. 4,870,329 to Ara, entitled Digital Convergence Circuit Storing Coefficients Of Fundamental Waves Of Correction Data, U.S. Pat. No. 3,854,108 to Horie et al., entitled Apparatus For Compensation Of Right And Left Pincushion Distortion, U.S. Pat. No. 3,819,979 to Truskalo, entitled High Voltage Regulators, U.S. Pat. No. 3,795,835 to Aldrich et al., entitled Horizontal Linearity Correction Circuit, U.S. Pat. No. 4,099,092 to Brinstow, entitled Television display Alignment System And Method, U.S. Pat. No. 4,707,640 to Onozawa et al., entitled Horizontal Deflection Output Circuit, U.S. Pat. No. 5,283,504 to Chauvin et al., entitled Grid Correction Circuit For TV Sets, U.S. Pat. No. 5,268,618 to Chen, entitled Linearity Compensation Method And Variable Magnetic Field Strength Linearity Compensation Apparatus For A Multi-Scanning Monitor, U.S. Pat. No. 5,420,484 to Morrish, entitled Deflection Apparatus For Raster Scanned CRT Displays, U.S. Pat. No. 5,416,389 to Merlo et al, entitled Horizontal Deflection Stage Linearity Control Device, U.S. Pat. No. 5,473,299 to Tsutsumi et al., entitled Horizontal Linearity Correction Coil, and U.S. Pat. No. 5,168,199 to Huffman et al., entitled Horizontal Linearity Correction Circuitry For Cathode Ray Tube Display.